Process of purifying and crystallizing salts



Patented Sept. 14, 1943 PROCESS OF PURIFYING AND CRYS TALLIZIN G SALTS Frederick J. Zimmermann, Wausau, Wis., as-' signor to Potash Company of America, Denver, 0010., a corporation of Colorado No Drawings Application July 15, 1940, Serial No. 345,611

(or. za zsm v amples of chemical salts particularly amenable 12 Claims.

This invention relates to a process of treating soluble chemical salts, and more particularly relates to controlled crystallization of such salts.

In the. manufacture of various chemical salts saturated solutions at atmospheric pressure. Ex-

to thetreatment are potassium chloride, potassium chlorate and sodium nitrate, although many other chemical salts likewise are amenable to for use as fertilizers or chemicals, substantial the treatment, as will be more specificallyset quantities of the product are obtained in the form forth hereinaftenf of particles of less than 80-mesh size. Such fine With the aforesaid essentials satisfied by'the materials tend to cake on standing and vare unproper selection of' the material" for treatment pleasant and difilcult to handle due to dusting. and the production of the saturated solution as V Also, where'used as fertilizers, the presence of leaforesaid, it will be apparent that even at the substantial quantities of such finely-divided maximum temperatures and pressures of the chemical salts makes their application to the soils treatment, some of the material will remain in 'dlfficult by reason of the aforementioned dusting ,SO p ase n he body Of'the P p and Such s d jconirlition; v constituents act as nuclei for the initiation of It is an object of the present invention to sub- 15 rystal f r t n p n Subsequent r u ion in ject chemical salts, particularly those of the fine temperature. l size range, to a recrystallization treatment which To this end, the pulp comprising the aforesaid V will increase the particle size and substantially ,,liquid and solid phas s-is subj e heat eliminate fines from the composition. treatment in a closed vessel to withstand the I Another object of' the invention is to provide a pressures generated as a result of e heating treatment for chemical salts in fine sizes, wherec ion nd h re y p v n n vap ra n acby the final product of such treatment will be a tion. In dissolving at atmosp pressure, composition of substantially uniform size and small quantities of soluble'impurities req e grade. e u siderable time to go into solution; At the in- A further object of the present invention is the creased temp r r s n in the added D provision of a treatment which will makea marsure generated, soluble impurities o into S0 11- ket product'of extremely fine sizes of chemical tion with much greater rap d ty- After heating salts occurring as dust-like materials. to the desired degree, the mixtureis subjected to Still another object of the invention is the .proacooling action, with the e u t particles o vision of a method for the productionof large the final product of the treatment form in sizes size particles of chemical salts of a high degree substantially a er t n 't Original Particles of purity. r t of the treatment and of substantially uniform Other objects reside in novel steps and treatsize. e ments, which will appearmore fully in' the course Thereafter, i s O y e s yl s p e e of the following description; liquid and solid phases by any suitable ,method,

In performing the present process, a soluble such as filtering, for example, and the solution so chemical salt occurring in finely-divided sizes, S p a ed y be recirculated 0 the p pfor example, predominantly less than IOO-mesh ingestage o t op ra nhe so s o t t 0 and usually having associated with it minor separated comprises the final product of the quantities of some other soluble salt as a con- 40 trea ment. I taminant or impurity, is taken as the material As a result of the foregoi g Oper D for treatment. A saturated solution of the comcedure, the-treatment will reduce the saturation position comprising the final product of the treatof the solution with a portion of the salt initially ment is initially produced and there is thenin; p s t liquid phase P i o o d Phase troduced into such saturated solution a quantity at the crystallization stage and being removed, of the material in excess of the amount which from the treatment with the solids content 7 will enter into solution at the highest tempera This condition serves, as a means 01' purifying ture at which the resulting pulp will be subjected the final product whenever'the contaminant is a during the period of treatment. soluble composition, as such soluble material will In order to attain the desired results,-it is necenter into solution during crystallization toreessary that the compositions treated shall be place the composition passing from the liquid those soluble chemical salts thatshow a substaninto the solid phase. tial increase in solubility with increase of tem- While a variety of sizes may be produced as a perature above the normal boiling points of their result of the aforesaidtreatment, it is possible in any given operation to so'regulate the treat- T OFFICE ment steps that the final product will be of substantially uniform particle size. The factors responsible for such control are the proportion of solid to solution in the material subjected to treatment, the rate of agitation during the heating stage, and the temperature range employed.

While it is possible to obtain beneficial results in the treatment of such materials where agitation is not employed, it has been found that agitation during both the heating and cooling stages is highly beneficial to the final results and consequently constitutes a preferred mode of operation.

In particular, the agitative action prevents caking of the solid phases and thereby insures sufficient dispersion of the fine particles throughout the liquid body to provide the proper nuclei for initiating particle formation. Further, th agitation serves to provide a more uniform heat exchange.

It also has been discovered that the rate of cooling is not a factor in the particle formation within the usual size range requiredjand because of'this, the preferred operating procedure upon reaching the maximum predetermined temperature is to resort to rapid cooling, inclusive of a sudden release of pressure in the closed container. Preferably, such released vapors are discharged into another container in which heating is progressing, and by so doing, a substantial heat 'loss is prevented, while the pressure release also serves to prevent incrustations of the salt crystals on the heat exchange surfaces. 1

This reduction in the cooling interval serves to shorten the treatment time of the operation and thereby increases plant capacity. In this connection, it will be understood that the pressure release into a second container in which heating is progressing serves to give a more rapid heat exchange and consequently shortens the heating interval.

Having thus described the operating procedure of the present invention, reference will be made to certain tests illustrating the efficiency of the present treatment.

In each of the tests shown in the following tabulation, the material taken for treatment comprised a potassium chloride concentrate from a mill in the Carlsbad, New Mexico, field-which analyzed approximately ninety-seven per cent KCl and approximately three per cent impurities consisting chiefiy of sodium chloride," but containing some minor inclusions of iron oxide, magnesium chloride and the like. The results of these tests follow: a

- Test N0. Filtered fine starting A B C D E material Pounds solid per gallon of brine l 7 6 5 4 Screen analyses accumulative, per cent:

From the foregoing, it will be observed that while the original solids content of the treatment consisted chiefly of extremely fine sizes, the final product of the treatment consisted principally of particles of uniform size. Further, in operations such as the foregoing, the grade of the product has uniformly improved from a purity of ninety-seven per cent to higher than ninety-nine per cent. 7

In such an operation, the continued dissolution of the sodium chloride content serves to gradually change the composition of the solution, and where continuous operation is employed, it will be necessary to'periodically withdraw the solution, deplete the potassium chloride content thereof and substitute fresh solution in place thereof. By a controlled cooling action, substantially all of the potassium chloride content of the solution may be recovered.

In certain operations, it may not be necessary to change the composition of the material under treatment and where two soluble salts are com- '--mingled in such material, the solution may be saturated with respect to both. Under such circumstances, the subsequent heating and cooling action will merely serve to alter particle sizes without changing the grade of the material.

While in theusual operation it is desirable ,to employ a rapid cooling action, ten to fifteen minutes, for example, it haslbeen discovered that larger crystals can be produced by a slow cooling action. While larger crystals are of no particular value'where the production is to be used in the fertilizer business, there way be occasions when such a product is desired. Therefore, the present process may be utilized to produce very large crystals by controlling the cooling action to materially extend its duration.

Due, to the fact that the'solution or brine is amenable to repeated usage, as previously explained, it is possible to employ the present process in both batch and continuous operations.

The solution separated from the final solids product at the end of the operation is recirculated usually through a storage tank and any dcpletion in saturation resulting from a given operation is taken care of by the addition of solids into such solutionat the head end of the operation. t

In this connection, it will be understood that periodic testing is employed to maintain the proper saturations and proper liquids-solids ratio at the head end of the'operation. When saturation of the solution drops, more solids will bemixed with the solution for a given treatment, and in this way, saturation during the operation is maintained substantially uniform.

As previously explained, a wide variety of chemical salts is amenable to the present treatment, and for convenience in the description and claims, these salts have been classified as soluble chemical salts, the solubility characteristics of which increase substantially above the normal boiling points at atmospheric pressure.

Various chlorides, chlorates, nitrates, sulfates, and bromides may be used, particularly those of sodium and potassium, but also including magnesium, such as potassium bromide, magnesium chloride and magnesium bromide, for example.

As previously stated, potassium chloride, potassium chlorate and sodium nitrate are particularly amenable to the treatment.

Where insoluble material is included as impurity, it will be understood that the dissolution and recrystallization action will not appreciably affect such constituents, but many oi'tli materials usually classified as insolubles, in reality, are only difiicultly soluble, and it will be understood that the relatively long interval of the heat treatment will besufficient to enter substantial quantities of such material into solution. In the subsequent recrystallization action producing the final prodnot, these constituents will not form in the crystals to any appreciable extent. Consequently, the

present process serves as a refining operation, in-

addition to controlling particle sizes. 7

While a considerable latitude is provided in the various control steps of the operation, best results have been obtained when the heating interval lasted for from two to five hours with a three to four hour interval as the preferred range.

Similarly, the temperature rise may vary to a considerable degree, but a temperature increase.

from 100" F. to not more than 600 F. due to the added pressureimposed, will be ample for most.

treatments and usually a maximum temperature of from 300 F. to 400 Fiis sufficient.

In this connection, it will be understood that inasmuch as best results are obtained when there is a relatively small quantity of solids present in the solution at the maximum temperature of the operation, excessive heating requires the presence of unusually large amounts of solids entered into the cold solution at the start of the operation. Therefore, it is necessary in order to determine the best operating procedure for a given treatment to consider the several factors of saturation. solubility characteristics of the final product, maximum temperature to be attained, and liquids-solids ratio, as well as the final particle size desired.

' The elimination of contaminating constituents benefits the final product by reducting its tendency to cake during storage and shipment. It is a well-known fact that two or more salts in combination are generally'more hygroscopic than either alone and the elimination of any appreciable quantity of the second salt from the saltcomprising the final product materially reduces the caking tendency.

Various means may be employed for providing the agitation, the heating actionand the cooling action of the operation. It has been found in practice'that an autoclave mounted for rotation and exteriorly heated, as by means of an oil flame, is well suited for the purposes of .the treatment. Cooling may be accelerated by release of the accumulated pressure, followed by spraying the exterior of the autoclave with water,'oi', if preferred, merely by atmospheric radiation. Preferably, some means of moving the charge, inaddition to therotation of the body, will be employed, such as lifter blades or projecting fins mounted on the interior walls of the cylinder.

The materials of the treatment may be taken from various sources, for example, the separated flnes of a flotation separation, the dusty constit-- solution of potassium chloride which was progressively heated in an autoclave, sealed from the atmosphere, to a maximum temperature of 200 C. and thereafter subjected to a progressive reduction in temperature for aninterval of several hours with crystals forming in sizes as large as inch at the completion of the cooling action.

From the foregoing, it is obvious that the present invention affords a wide latitude in the type'of product produced to satisfy a variety of conditions. As the factors controlling grain size, namely, '(1) proportion of solidmaterial introduced into the solution, (2) the speed'of agitation, and (3) the temperature range employed, may be individually varied, any predetermined grain size within the limits indicated herein may be attained by following the methods of. the present invention. In all such treatments, the substantial quantities of extremely fine sizes forming the pulp at the beginning of the operation are converted by'the process into grains of sufficient size, to be usable in the various requirements of commerce.

. Changes and modifications may be availed of within the spirit and scope of the invention as defined in the hereunto appended claims.

What I claim and desire to secure by Letters Patent is:

1. A process of treating soluble chemical salts, the. solubility of which increases substantially at temperatures above those of. their saturated solutions at their normal atmospheric boiling points, which comprises introducing such a chemical salt in finely-divided condition into a saturated solutionof such salt to form a pulp, heating such pulp while sealed from the atmosphere under pressures in excess of normal atmospheric conditions to a temperature above the boiling point of its liquid phase at atmospheric pressure but less than the temperature necessary to dissolve, all the solids of the pulp, inducing a crystal formation on the solids nuclei remaining at the said higher temperature by cooling the pulp, and separating the resultant solids from the liquid at the completion of the cooling action.

2. A process of treating soluble chemical salts. V the solubility of which increases substantially at temperatures above those of their saturated solutions at their normal atmospheric boiling 7 by cooling the pulp, and separating the resultant solids from. the liquid at the completion oi the cooling action.

3. A process of treating soluble chemical salts, the solubility 01' which increases substantially at temperatures above those of their saturated solutions at their normal atmospheric boiling points, which comprises introducing such' a chemical salt in finely-divided condition into a saturated solution of such salt to form apulp. in the approximate proportions of four to twelve pounds ofthe. salt per gallon of saturated solution,.heating such pulp while sealed from the atmosphere under pressure in excess of normal atmospheric conditions to a temperature above the boiling point of its liquid phase at atmospheric pressure but less than the temperature necessary to dissolve all the solids of the pulp,

saturated solution of such salt to form a pulp.

heating said pulp while sealed'from the atmosphere under pressures in excess of normal atmospheric conditions to a maximum temperature between 300 F. and 400 F., inducing a crystal formation on the'solids nuclei remaining in the pulp at said maximum temperature by cooling the pulp, and separating the resultant solids from the liquid at cooling action.

5. A process of treating soluble chemical salts, the solubility of which increases substantially at temperatures above those of their saturated solutions at their normal atmospheric boiling points, which comprises introducing such a chemical salt in finely-divided condition into a the completion of the saturated solution of such salt to form a pulp,

heating such pulp while sealed from the atmosphere under pressures in excess of normal atmospheric conditions to a temperature above the boiling point of its liquid phase at'atmospheric pressure-but less than the temperature necessary to dissolve all the solids of the pulp, exhausting the resulting vapors from the sealed zone, inducing a crystal formation on the solids nuclei remaining at the said higher temperature by cooling the pulp, and separating the resultant solids from the liquid at the completion of the cooling action. 7

6. A process of treating potassium chloride containing some sodium chloride as an impurity,'

which comprises introducing such a composition in firiely-divided condition into a saturated potassium chloride solution to form a pulp inan autoclave, heating said pulp while sealed from the atmosphere to a temperature above th boiling point of its liquid phase at atmospheric pressure but less than thetemperature necessary to dissolve all potassium chloride solids of the pulp, inducing a crystal formation on the potassium chloride nuclei remaining in the pulp at the said higher temperature by cooling the pulp and thereby dissolving the sodium chloride solids, and separating the resultant potassium chloride solids from the liquid phase at the completion of the cooling action.

'7. In a process of treating a potassium chloride composition, the improvement which comprises introducing such a composition in finelydivided condition and in a quantity in excess of the amount entering into solution at the maximum temperature of the treatment, into a saturated potassium chloride solution to form a pulp, heating said pulp in an autoclave to a temperature in excess of'that attainable at atmospheric pressure, and inducing a crystal formation on undissolved potassium chloride of the heating action by cooling the heated pulp.

8. In a process of treating a potassium chlo' rate composition, the improvement which com prises introducing such a composition in finelydivided condition and in a quantity in excess of pressure, and inducing a crystal formation on undissolved potassium chlorate of the heating action by cooling the heated pulp.

9. In a process of treating a sodium'nitrate composition, the improvement which comprises introducing such a composition in finely-divided condition and in a quantity in excess of the amountentering into solution at the maximum temperature of the treatment, into a saturated sodium nitrate solution to form a pulp, heating said pulp in an autoclave to a temperature in excess of that attainable at atmosphericpressure, and inducing a crystal formation on undissolved sodium nitrate of the heating'ac'tion by cooling the heated pulp.

10. A process of treating soluble chemical salts, the solubility of which increases substantially at temperatures above those of their saturated solutions at their normal atmospheric boiling points,

a which comprises introducing such a chemical salt in'finely-divided condition into a saturated solution of such salt to form a pulp, heating such pulp while sealed from the atmosphere under pressures in excess of normal atmospheric conditions to a temperature above the boiling point of its liquid phase at atmospheric pressure but less than the temperature necessary to dissolve all the solids of the pulp, inducing a crystal fo mation on the solids nuclei remaining at the said higher temperature by cooling the pulp, separating the resultant solids from the liquid at the completion of the cooling action, and returning the solution so separated to the pulp-forming stage of the operation.

11. In a process of treating soluble chemical salts, the solubility of which increases substantially at temperatures above those of their saturated solutions at their normal atmospheric boiling points, the improvement which comprises introducing such a chemical salt in finely-divided condition and in a quantity in excess of the amount entering into solution at the maximum temperature of the treatment, into a saturated solution of such salt to form a pulp, heating such pulp in an autoclave to temperatures in excess of those attainable 'at atmospheric pressure, and inducing a crystal formation on undissolved solids of the heating action by cooling the heated pulp.

12. In 'a process of treating soluble chemical salts, the solubility of which increases substantially at temperatures above those of their saturated solutions at their normal atmospheric boiling points, including the improvement which comprises introducing such a chemicatsalt consisting principally of particles finer than 80 mesh size into a saturated solution of such salt to form a-pulp, heating such pulp while sealed from the atmosphere under pressures in excess of normal atmospheric conditions to temperatures in excess of 200 C. while maintaining some undissolved solids of the salt in dispersed condition in the heatedbody. and inducing crystal formation in sizes approximating inch byprogressively cooling the heated mass for an interval in excess of one hour.

FREDERICK J. ZIMMERMANN.

ture in excess of that attainable at atmospheric 7' 

